It has long been established that enriched oxygen air can be a beneficial aid for certain medical treatments and also that enriched oxygen air can improve the efficiency of various industrial processes. The more widespread use of oxygen rich air is, however, dependent on whether or not oxygen rich air can be supplied in a cost effective manner. Many potential applications, particularly industrial processes, usually also require large amounts of enriched oxygen air, and large volumes can only be supplied commercially, at the present time, by diluting pure or almost pure oxygen with normal atmospheric air. Pure oxygen can of course be supplied by oxygen manufacturers, as either compressed oxygen or liquid oxygen, however, the amount that can be supplied in compressed gas or liquid form is limited and the oxygen is also extremely expensive.
For large processes requiring high volumes of oxygen, the only practical alternative is to produce pure oxygen on-site by an industrial method of manufacture, such as pressure swing adsorption, vacuum swing adsorption or a cryogenic system. However, the oxygen would still be expensive, because of the high capital and energy costs associated with these methods of manufacturing oxygen. Industrial scale oxygen production units also require a large amount of space. The on-site manufacture of oxygen is therefore only realistic for industries, such as the metal and petroleum industries, which have processes large enough to have the economy of scale to justify an oxygen production plant.
Oxygen concentrators, based on membrane gas separation systems, can be used to produce enriched oxygen air. Most commercial oxygen concentrators tend to have high gas selectivity but relatively low gas permeability. Although these oxygen concentrators are able to produce reasonably pure gas streams, they generally operate at high pressures and they are usually only able to produce relatively small volumes of separated gases. Because of their high-pressure operation, these oxygen concentrators have high demands for energy. The membranes used in these types of oxygen concentrator are also prone to failure, because continual operation under high-pressure places considerable stress on the membranes.
To satisfy the potential medical and industrial applications that exist for enriched oxygen air, a low-pressure, energy efficient gas separation membrane system, which is able to produce large volumes of cost effective enriched oxygen air, is required. A typical composite hollow fibre gas separation membrane consists of two basic components, an asymmetric hollow fibre tube, which forms the porous support structure of the membrane, and a coating of a dense polymer on the outside surface of the fibre tube, which provides the gas selectivity properties of the membrane.
The gas separation performance of a composite hollow fibre membrane is therefore very dependent on the porosity of the asymmetric fibre support and on the thickness of the selective polymer layer coated onto the outside of the tube.
For example, the porous hollow fibre tube has to provide mechanical support for the selective layer; have an open porous cell structure to minimise resistance to gas transmission across the fibre tube; have no voids in the structure; and preferably have no closed pores within the structure.
The selective top layer has to be as thin as possible; be of reasonably uniform thickness; be essentially free of holes and defects; and not be so thick as to plug open pores on the outer surface of the fibre support.
An open, porous cell network in the asymmetric fibre support is essential to provide high gas transfer and permeability of gases across the hollow fibre tube. However, a very porous fibre structure, as would be expected, has poor selectivity between different gases, and the selectivity properties have to be provided by depositing a coating of dense selective polymer onto the outside surface of the fibre tube. However, the gas separation performance of the membrane is very dependent on the thickness and the quality of the selective coating deposited onto the hollow fibre support.
For example, if the open exposed pores on the outside surface of the fibre substrate are very large, it is difficult to deposit a defect free layer of the selective polymer onto the fibre support, and any holes or ruptures in the coating would have a very detrimental effect on the selective properties of the membrane.
It is also essential that the selective layer is as thin as possible, in order to provide a reasonable degree of gas permeability through the coating as well as gas selectivity. Increasing the thickness of the selective layer, to cover large open pores in the outer surface of the substrate, merely reduces the permeability of the gas separation membrane. A thick selective coating would also significantly increase the pressure differential required to effect gas separation, as gas transport through the dense selective layer is a major rate-determining step. Excessive thickening of the selective top layer can also lead to dense polymer material penetrating into the open pores of the fibre support, rather than lying on the surface of the substrate, and plugged pores can also significantly increase the resistance to gas flow through the membrane.
The practical performance of a composite hollow fibre gas separation membrane is therefore dependent on having an appropriate balance between a very porous, highly permeable fibre support and a very thin, uniform, defect free selective layer on the outer surface of the fibre support.